1. Field of the Art
The present invention relates to a structure mounting a polygon mirror on a rotor of a polygon mirror scanner motor.
2. Prior Art
A polygon mirror mounted on a rotor of a polygon mirror scanner motor is known.
FIG. 10 is a sectional view showing the structure of a rotor of a conventional polygon mirror scanner motor of the type described above. In FIG. 10, reference numeral 51 denotes a cylindrical ceramic ring. A yoke (sleeve) 52 is shrink-fitted to the ceramic ring 51. Thereafter, surfaces of the yokes (sleeves) 52 and 55 which are to come into contact with a polygon mirror 53 are finished with extremely accurate flatness and surface roughness. Then, the polygon mirror 53 is held between the yokes 52 and 55 and firmly fastened by a plurality of screws 66. In addition, a rotor magnet 56 of the motor is rigidly secured to the lower surface of the yoke 55. It should be noted that the polygon mirror 53 has a polygonal plane, and a number of mirror surfaces 54 which is equal to the number of sides of the polygonal plane are formed on the outer periphery of the polygon mirror 53.
FIG. 11 shows a section of a conventional polygon mirror scanner motor having the above-described rotor incorporated therein. As illustrated in the figure, the polygon mirror scanner motor includes a mounting plate 57. A support shaft 58 is secured to the mounting plate 57, and a ceramic radial shaft 59 is secured to the support shaft 58. A ceramic thrust plate 61 is secured to the lower end surface of the radial shaft 59. The above-described rotor is disposed in such a manner that the ceramic ring 51 is in sliding contact with the outer periphery of the radial shaft 59, and the lower end surface of the ceramic ring 51 is in sliding contact with the thrust plate 61. Another ceramic thrust plate 60 is secured to the lower surface of a support plate 67. The thrust plate 60 is in contact with the upper end surface of the radial shaft 59. The outer peripheral surface of the ceramic radial shaft 59 and the inner peripheral surface of the ceramic ring 51 constitute a radial hydrodynamic bearing. Meanwhile, thrust hydrodynamic bearings are composed of the respective surfaces of the ceramic thrust plates 60 and 61, which are secured to the upper and lower end surfaces of the radial shaft 59, and the upper and lower surfaces of the ceramic ring 51.
The support plate 67 is secured to the upper end of the support shaft 58 by using a bolt 62. A counter magnet 63 is secured to a predetermined position on the lower surface of the support plate 67 to levitate the yoke 52, which is made of a magnetic material, by a predetermined magnetic force. By supplying a driving current (AC) to a stator coil 64, the rotor, having the polygon mirror 53, is driven to rotate at high speed.
However, the above-described conventional polygon mirror mounting structure suffers from the following problems:
(1) The polygon mirror 53 must be provided with several through-holes for receiving the screws 66. Therefore, when the polygon mirror 53 is rotated at high speed, there is a difference in the radial deformation of the polygon mirror 53, which is caused by centrifugal force and heat, between a portion where a through-hole is provided and a portion where no through-hole is provided. As a result, non-uniform deformation occurs, and hence non-uniform deformation is induced for each of the mirror surfaces 54. The non-uniform deformation in the polygon mirror exerts an adverse effect such as bad Dynamic Tracking Jitter or low Dynamic Tracking Accuracy. PA1 (2) The polygon mirror 53 is rigidly fastened to the yokes 52 and 55 by several screws 66, and the fastening is controlled by only the surface friction occurring between the contact surfaces of the yokes 52 and 55 and the polygon mirror 53. Accordingly, if there is a temperature change, e.g., a rise in temperature during rotation at high speed or a lowering in temperature during storage, since the polygon mirror 53 is made of an aluminum material while the yokes 52 and 55 are made of a steel material, a large difference in the coefficients of thermal expansion causes slippage to occur between the contact surfaces of the polygon mirror 53 and the yokes 52 and 55. Such slippage exerts an adverse effect on the mirror surfaces 54, causing bad Dynamic Tracking Jitter or low Dynamic Tracking Accuracy over time and also results in the rotor becoming unbalanced due to relative radial displacement between the polygon mirror and the yokes, resulting in an increase in the magnitude of vibration. PA1 (3) Since the performance is mostly determined by the surface friction occurring between the contact surfaces of the polygon mirror 53 and the yokes 52 and 55, it is necessary to finish the surfaces of the yokes 52 and 55 which are to come in contact with the polygon mirror 53 and the surfaces of the polygon mirror 53 which are to come in contact with the two yokes 52 and 55, that is, a total of 4 surfaces, with a highly accurate degree of flatness and surface roughness over a considerably wide area, thus causing a rise in processing costs. PA1 (4) Since the height of the yoke 52 which is to be shrink-fitted to the ceramic ring 51 is lower than that of the ceramics ring 51, the yoke 52 is shrink-fitted only to an upper portion of the ceramic ring 51. Accordingly, vertically non-uniform residual stress is applied to the ceramic ring 51 after the yoke 52 has been shrink-fitted thereto. Consequently, the upper and lower end surfaces of the ceramic ring 51 are largely deformed. Therefore, it has heretofore been necessary to carry out correction-processing after the shrink fitting process. PA1 (5) When screws are fastened to the yoke 55, the portions of the yoke 55 near the screws are deformed, thus deforms the reference surface of the polygon mirror 53 contacting the yoke surface. Further, since several screws are used for fastening, the fastening order of the screws, an additional fastening (or twice fastening) of the screws and the torque for fastening must be controlled, which complicates the assembly operation and increases man-hours. PA1 (6) Rotor magnet 56 is secured to the lower surface of the yoke 55 by an adhering agent. However, the adhering agent often enters the screw holes which prevents complete fastening of the screws. PA1 (7) In the conventional structure, the polygon mirror 53, the yoke 55 and the rotor magnet 56 are suspended from the sleeve 52, which is shrink-fitted to the ceramic ring 51, by using the screws 66. Therefore, the overall weight of the suspended members is considerably greater than the weight of the sleeve 52, and during high-speed rotation, the suspended members, which have such a large overall weight, are rotated relying only on the support of the screws 66. Accordingly, the stability of the rotor is unsatisfactory, and balance deteriorates with time, resulting in an increase in the magnitude of vibration.